Touch system and positioning method therefor

ABSTRACT

The present invention provides a positioning method for a touch system that obtains a pair of current correct positions according to the following steps: obtaining two pairs of possible positions from a current frame to be compared with a pair of previous correct positions obtained from a previous frame; or comparing four pairs of possible positions with each other obtained from the current frame. The present invention further provides a touch system.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan PatentApplication Serial Number 099119224, filed on Jun. 14, 2010, the fulldisclosure of which is incorporated herein by reference.

BACKGROUND

1. Field of the Invention

This invention generally relates to a touch system and, moreparticularly, to an optical touch system and a positioning methodtherefor.

2. Description of the Related Art

Please referring to FIGS. 1 a and 1 b, FIG. 1 a shows a schematicdiagram of an optical touch system and FIG. 1 b shows a schematicdiagram of image windows acquired by the two image sensors included inthe touch system shown in FIG. 1 a.

The touch system 9 includes a touch surface 90 and two image sensors 91and 91′. The image sensors 91 and 91′ are configured to acquire imagewindows W₉₁ and W₉₁′ respectively looking across the touch surface 90.When a finger 81 is hovering above or touches the touch surface 90, theimage windows W₉₁ and W₉₁′ acquired by the image sensors 91 and 91′respectively include finger images I₈₁ and I₈₁′ of the finger 81. Aprocessing unit 92 can calculate a two-dimensional coordinate of thefinger 81 with respect to the touch surface 90 according to aone-dimensional position of the finger image I₈₁ in the image window W₉₁and a one-dimensional position of the finger image I₈₁′ in the imagewindow W₉₁′.

However, when a plurality of fingers are hovering above or touch thetouch surface 90 simultaneously, one finger may block other finger orfingers with respect to a part of the image sensors. For example in FIG.1 a, when two fingers 81 and 82 are hovering above or touch the touchsurface 90, the image sensor 91 acquires images of the fingers 81 and 82following a route “a” and the image sensor 91′ acquires images of thefingers 81 and 82 following routes “b” and “c” respectively. To theimage sensor 91, as the finger 81 blocks the finger 82, the image windowW₉₁ acquired by the image sensor 91 only includes a finger image I₈₁+I₈₂(i.e. a combined image of the finger images I₈₁ and I₈₂). Therefore, theprocessing unit 92 is not able to correctly calculate two-dimensionalcoordinates of different fingers with respect to the touch surface 90according to the finger images I₈₁′, I₈₂′ and I₈₁+I₈₂.

Accordingly, it is necessary to provide a positioning method for anoptical touch system that can correctly position a plurality ofpointers.

SUMMARY

The present invention provides a touch system and a positioning methodtherefor configured to correctly obtain two-dimensional coordinates of aplurality of pointers with respect to a touch system.

The present invention provides a positioning method for a touch system.The touch system includes a first image sensor and a second image sensorfor acquiring image windows looking across a touch surface andcontaining images of two pointers operating above the touch surface. Thepositioning method includes the steps of: acquiring a first image windowwith the first image sensor; acquiring a second image window with thesecond image sensor; identifying numbers of pointer images in the firstimage window and the second image window; generating a two-dimensionalspace according to the first image window and the second image windowwhen the first image window and the second image window containdifferent numbers of pointer images; connecting, on the two-dimensionalspace, a mapping position of the first image sensor with mappingpositions of two outermost edges of the pointer image in the first imagewindow and connecting, on the two-dimensional space, a mapping positionof the second image sensor with mapping positions of two outermost edgesof the pointer image in the second image window to form a quadrilateral;calculating four first internal bisectors of the quadrilateral; andconnecting, on the two-dimensional space, a mapping position of theimage sensor acquiring more pointer images with mapping positions of apredetermined point of the pointer images in the image window acquiredby the same image sensor to intersect with the first internal bisectorsthereby generating first possible positions.

In another aspect, a pair of previous correct positions of the pointerswith respect to the touch surface was determined in a previous sampletime (image capture time) before the first image sensor acquires thefirst image window and the second image sensor acquires the second imagewindow. The positioning method further includes the steps of: comparingthe first possible positions with the pair of previous correct positionsto obtain a pair of current correct positions.

In another aspect, the touch system further includes a third imagesensor for acquiring image windows looking across the touch surface andcontaining images of the two pointers. The positioning method furtherincludes the steps of: acquiring a third image window with the thirdimage sensor; identifying the numbers of pointer images in the first,second and third image windows; mapping the third image window to thetwo-dimensional space when the numbers of pointer images in two of theimage windows are smaller than that in the rest image window;connecting, on the two-dimensional space, mapping positions of two imagesensors acquiring fewer pointer image with mapping positions of twooutermost edges of the pointer image in the image windows acquired bythe same two image sensors to form a quadrilateral; calculating foursecond internal bisectors of the quadrilateral; connecting, on thetwo-dimensional space, a mapping position of the image sensor acquiringmore pointer images with mapping positions of a predetermined point ofthe pointer images in the image window acquired by the same image sensorto intersect with the second internal bisectors thereby generatingsecond possible positions; and comparing the first possible positionswith the second possible positions to obtain a pair of current correctpositions.

In another aspect, the touch system further includes a third imagesensor for acquiring image windows looking across the touch surface andcontaining images of the two pointers. The positioning method furtherincludes the steps of: acquiring a third image window with the thirdimage sensor; identifying the numbers of pointer images in the first,second and third image windows; mapping the third image window to thetwo-dimensional space when the numbers of pointer images in two of theimage windows are larger than that in the rest image window; connecting,on the two-dimensional space, a mapping position of one of two imagesensors acquiring more pointer images with mapping positions of twooutermost edges of the pointer images in the image window acquired bythe same image sensor and connecting, on the two-dimensional space, amapping position of the image sensor acquiring fewer pointer image withmapping positions of two outermost edges of the pointer image in theimage window acquired by the same image sensor to form a quadrilateral;calculating four third internal bisectors of the quadrilateral;connecting, on the two-dimensional space, a mapping position of one oftwo image sensors acquiring more pointer images with mapping positionsof a predetermined point of the pointer images in the image windowacquired by the same image sensor to intersect with the third internalbisectors thereby generating third possible positions; and comparing thefirst possible positions with third possible positions to obtain a pairof current correct positions.

In another aspect, the touch system further includes a third imagesensor for acquiring image windows looking across the touch surface andcontaining images of the two pointers. The positioning method furtherincludes the steps of: acquiring a third image window with the thirdimage sensor; identifying the numbers of pointer images in the first,second and third image windows; mapping the third image window to thetwo-dimensional space when the numbers of pointer images in two of theimage windows are larger than that in the rest image window; connecting,on the two-dimensional space, mapping positions of two image sensorsacquiring more pointer images with mapping positions of a predeterminedpoint of the pointer images in the image windows acquired by the sametwo image sensors to form a quadrilateral; defining four corners of thequadrilateral as fourth possible positions of the pointers with respectto the touch surface; and comparing the first possible positions withthe fourth possible positions to obtain a pair of current correctpositions.

The present invention further provides a positioning method for a touchsystem. The touch system includes a first image sensor, a second imagesensor and a third image sensor for acquiring image windows lookingacross a touch surface and containing images of two pointers operatingabove the touch surface. The positioning method includes the steps of:respectively acquiring an image window with three image sensors;identifying numbers of pointer images in the image windows; generating atwo-dimensional space according to the three image windows; executingthe following steps when the numbers of pointer images in two of theimage windows is smaller then that in the rest image window: connecting,on the two-dimensional space, mapping positions of two image sensorsacquiring fewer pointer image with mapping positions of two outermostedges of the pointer image in the image windows acquired by the same twoimage sensors to form a quadrilateral; calculating four second internalbisectors of the quadrilateral; and connecting, on the two-dimensionalspace, a mapping position of the image sensor acquiring more pointerimages with mapping positions of a predetermined point of the pointerimages in the image window acquired by the same image sensor tointersect with the second internal bisectors thereby generating secondpossible positions; and executing the following steps when the numbersof pointer images in two of the image windows is larger then that in therest image window: connecting, on the two-dimensional space, a mappingposition of one of two image sensors acquiring more pointer images withmapping positions of two outermost edges of the pointer images in theimage window acquired by the same image sensor and connecting, on thetwo-dimensional space, a mapping position of the image sensor acquiringfewer pointer image with mapping positions of two outermost edges of thepointer image in the image window acquired by the same image sensor toform a quadrilateral; calculating four third internal bisectors of thequadrilateral; and connecting, on the two-dimensional space, a mappingposition of one of two image sensors acquiring more pointer images withmapping positions of a predetermined point of the pointer images in theimage window acquired by the same image sensor to intersect with thethird internal bisectors thereby generating third possible positions.

In another aspect, a pair of previous correct positions of the pointerswith respect to the touch surface was determined in a previous sampletime (image capture time) before the image sensors acquire the imagewindows. The positioning method further includes the steps of: comparingthe second possible positions with the pair of previous correctpositions to obtain a pair of current correct positions when the numbersof pointer images in two of the image windows are smaller than that inthe rest image window; and comparing the third possible positions withthe pair of previous correct positions to obtain a pair of currentcorrect positions when the numbers of pointer images in two of the imagewindows are larger than that in the rest image window.

In another aspect, the positioning method further includes the steps of:selecting two image sensors acquiring different numbers of pointerimages; connecting, on the two-dimensional space, mapping positions ofthe two image sensors respectively with mapping positions of twooutermost edges of the pointer images in the image windows acquired bythe same two image sensors to form a quadrilateral; calculating fourfirst internal bisectors of the quadrilateral; connecting, on thetwo-dimensional space, a mapping position of one of the two imagesensors acquiring more pointer images with mapping positions of apredetermined point of the pointer images in the image window acquiredby the same image sensor to intersect with the first internal bisectorsthereby generating first possible positions, wherein comparing thesecond possible positions with the first possible positions to obtain apair of current correct positions when the numbers of pointer images intwo of the image windows are smaller than that in the rest image window,and comparing the third possible positions with the first possiblepositions to obtain a pair of current correct positions when the numbersof pointer images in two of the image windows are larger than that inthe rest image window.

In another aspect, when the numbers of pointer images in two of theimage windows are larger than that in the rest image window, thepositioning method further includes the steps of: connecting, on thetwo-dimensional space, mapping positions of two image sensors acquiringmore pointer images with mapping positions of a predetermined point ofthe pointer images in the image windows acquired by the same two imagesensors to form a quadrilateral; defining four corners of thequadrilateral as fourth possible positions; and comparing the thirdpossible positions with the fourth possible positions to obtain a pairof current correct positions.

The present invention further provides a touch system including a touchsurface, at least two image sensors and a processing unit. A pluralityof pointers are operated above the touch surface to accordingly controlthe touch system. The image sensors are configured to acquire imagewindows looking across the touch surface and containing images of thepointers operating above the touch surface. The processing unitgenerates a two-dimensional space according the image windows acquiredby the image sensors, obtains a quadrilateral and four internalbisectors of the quadrilateral by connecting mapping positions of theimage sensors with mapping positions of two outermost edges of thepointer image in the image windows acquired by the image sensors on thetwo-dimensional space, and connects a mapping position of the imagesensor acquiring more pointer images with mapping positions of apredetermined point of the pointer images in the image window acquiredby the same image sensor to intersect with the internal bisectorsthereby generating possible positions.

In the touch system and positioning method therefore of the presentinvention, when the number of pointer images in the image windowacquired by at least one image sensor is equal to an actual number ofthe pointers, correct positions of all pointers still can be calculatedeven though the number of pointer images in the image window acquired bythe rest image sensor is smaller than the actual number of the pointers.

In the touch system and positioning method therefore of the presentinvention, two-dimensional information, such as two-dimensionalcoordinates, edge lines, position lines and internal bisectors,processed by the processing unit is mapped from the one-dimensionalimage windows acquired by a plurality of image sensors, wherein theinternal bisectors may be calculated by using vector arithmetic from thefour sides of the quadrilateral.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, advantages, and novel features of the present inventionwill become more apparent from the following detailed description whentaken in conjunction with the accompanying drawings.

FIG. 1 a shows a schematic diagram of an optical touch system.

FIG. 1 b shows a schematic diagram of image windows acquired by theimage sensors included in the touch system shown in FIG. 1 a.

FIG. 2 a shows a schematic diagram of the touch system according to thefirst embodiment of the present invention.

FIG. 2 b shows a schematic diagram of image windows acquired by theimage sensors included in the touch system according to the firstembodiment of the present invention.

FIG. 2 c shows a schematic diagram of the positioning method for thetouch system according to the first embodiment of the present invention.

FIG. 2 d shows a flow chart of the positioning method for the touchsystem according to the first embodiment of the present invention.

FIG. 3 shows a schematic diagram of the touch system according to thesecond embodiment of the present invention.

FIG. 4 a shows a schematic diagram of the positioning method for thetouch system according to a first aspect of the second embodiment of thepresent invention.

FIG. 4 b shows a flow chart of the positioning method for the touchsystem according to the first aspect of the second embodiment of thepresent invention.

FIG. 4 c shows a schematic diagram of the positioning method for thetouch system according to a second aspect of the second embodiment ofthe present invention.

FIG. 4 d shows a flow chart of the positioning method for the touchsystem according to the second aspect of the second embodiment of thepresent invention.

FIGS. 5 a to 5 d show schematic diagrams of the positioning method forthe touch system according to a third aspect of the second embodiment ofthe present invention.

FIG. 5 e shows a flow chart of the positioning method for the touchsystem according to the third aspect of the second embodiment of thepresent invention.

FIG. 6 a shows a schematic diagram of the positioning method for thetouch system according to a fourth aspect of the second embodiment ofthe present invention.

FIG. 6 b shows a flow chart of the positioning method for the touchsystem according to the fourth aspect of the second embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE EMBODIMENT

It should be noted that, wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

In addition, only a part of components are shown in the drawings and thecomponents that are not directly pertinent to the present invention areomitted.

A touch system of the present invention includes at least two imagesensors. A positioning method for the touch system is applicable to atouch system controlled by a user (not shown) with a plurality ofpointers and in the touch system one pointer blocks another pointer withrespect to at least one image sensor. That is, numbers of pointer imagescontained in the image windows acquired by the plurality of imagesensors are different from actual numbers of the pointers. In addition,when the numbers of pointer images contained in the image windowsacquired by the plurality of image sensors is equal to the actualnumbers of the pointers, the two-dimensional coordinate of every pointermay be traced by using other conventional methods.

Please referring to FIG. 2 a, it shows a schematic diagram of the touchsystem according to the first embodiment of the present invention. Thetouch system 1 includes a touch surface 10, a first image sensor 11, asecond image sensor 11′ and a processing unit 12. The touch surface 10may be a white board, a touch screen or the surface of a suitableobject. When the touch surface 10 is a touch screen, the touch surface10 may also configured to display the operation status, such as themotion of a cursor or a predetermined function (e.g. screen rolling,object zooming or the like). In addition, the touch system 1 may furtherinclude a display for displaying the operation status.

The first image sensor 11 and the second image sensor 11′ may be, forexample CCD image sensors, CMOS image sensors or the like, and areconfigured to synchronously acquire an image window looking across thetouch surface 10 within each image capture time. The image sensors mayhave the ability of blocking visible lights so as to eliminate theinterference from ambient lights; for example, but not limited to, anoptical bandpass filter may be disposed in front of the image sensors.The processing unit 12 is configured to process the image windowsacquired by the first image sensor 11 and the second image sensor 11′,and to trace and to position the pointers, such as to calculate thetwo-dimensional coordinates of the pointers 81 and 82 with respect tothe touch surface 10. In this invention, the pointer may be a finger, atouch pen, a rod or other suitable objects. It is appreciated that,locations of the first image sensor 11 and the second image sensor 11′are not limited to those shown in FIG. 2 a. For example, the first imagesensor 11 may be disposed at the lower left corner and the second imagesensor 11′ may be disposed at the lower right corner.

Please referring to FIG. 2 b, it shows an image window W₁₁ acquired bythe first image sensor 11 and an image window W₁₁′ acquired by thesecond image sensor 11′ shown in FIG. 2 a. The image window W₁₁ containstwo pointer images I₈₁ and I₈₂ respectively corresponding to thepointers 81 and 82, and has a numerical range, such as 0 to 960, to forma one-dimensional space. The image window W₁₁′ contains a pointer imageI′ (combined image) corresponding to the pointers 81 and 82, and has anumerical range, such as 0 to 960, to form another one-dimensionalspace. It is appreciated that, the numerical range may be determined byan actual size of the touch surface 10.

To the second image sensor 11′, as the pointer 81 blocks the pointer 82,the image window W₁₁′ in FIG. 2 b includes only one pointer image I′.According to the one-dimensional numerical ranges of the image windowsW₁₁ and W₁₁′, a two-dimensional space S (as shown in FIG. 2 c) can bemapped and the two-dimensional space S is corresponding to the touchsurface 10. In other words, a pair of numerical numbers of the imagewindows W₁₁ and W₁₁′ corresponds to a two-dimensional coordinate on thetwo-dimensional space S. For example, (W₁₁,W₁₁′)=(0,0) corresponds tothe upper left corner of the two-dimensional space S and(W₁₁,W₁₁′)=(960,960) corresponds to the lower right corner of thetwo-dimensional space S, but the present invention is not limitedthereto. A corresponding relationship between a pair of numericalnumbers of the image windows and a two-dimensional coordinate may bedetermined according to the actual application.

The positioning method of every embodiment or aspect of the presentinvention may be implemented by performing two-dimensional coordinateoperation and vector arithmetic on the two-dimensional space S.

Please referring to FIGS. 2 a to 2 d, FIG. 2 d shows a flow chart of thepositioning method for a touch system according to the first embodimentof the present invention including the steps of: acquiring a first imagewindow with a first image sensor (Step S₁₀); acquiring a second imagewindow with a second image sensor (Step S₁₁); identifying numbers ofpointer images in the first image window and the second image window(Step S₁₂); generating a two-dimensional space according to the firstimage window and the second image window when the first image window andthe second image window contain different numbers of pointer images(Step S₁₃); connecting, on the two-dimensional space, a mapping positionof the first image sensor with mapping positions of two outermost edgesof the pointer image in the first image window to form a first edge lineand a second edge line, and connecting, on the two-dimensional space, amapping position of the second image sensor with mapping positions oftwo outermost edges of the pointer image in the second image window toform a third edge line and a fourth edge line (Step S₁₄); calculatingfour internal bisectors of a quadrilateral formed by the first edge lineto the fourth edge line (Step S₁₅); connecting, on the two-dimensionalspace, a mapping position of the image sensor acquiring more pointerimages with mapping positions of a predetermined point of the pointerimages in the image window acquired by the same image sensor to form twofirst position lines (Step S₁₆); defining cross points of the firstposition lines and the first internal bisectors as first possiblepositions (Step S₁₇); and comparing the first possible positions with apair of previous correct positions to obtain a pair of current correctpositions (Step S₁₈), wherein the pair of previous correct positions isdetermined in a previous image capture time (sample time) of the firstimage sensor and the second image sensor. In addition, before comparingthe first possible positions and a pair of previous correct positions,two possible positions associated with the two internal bisectors of twoopposite corners of the quadrilateral may be defined as a pair of firstpossible positions, and each pair of the first possible positions isthen compared with the pair of previous correct positions pair by pair.

The image sensors 11 and 11′ respectively acquire an image window W₁₁and W₁₁′ at a sample time “t”, and one of the image windows W₁₁ and W₁₁′includes only one pointer image (Steps S₁₀, S₁₁). In the meantime, it isassumed that both image windows W₁₁ and W₁₁′ respectively acquired bythe image sensors 11 and 11′ at a sample time “t−1” include two pointerimages. That is, one of the pointers 81 and 82 does not block the otherwith respect to any image sensor at the sample time “t−1”.

Please referring to FIGS. 2 a to 2 c, the processing unit 12 processesthe image windows W₁₁ and W₁₁′ so as to identify whether the imagewindows W₁₁ and W₁₁′ contain an identical number of pointer images (StepS₁₂). When the processing unit 12 identifies that the first image windowW₁₁ and the second image window W₁₁′ contain different numbers ofpointer images, the processing unit 12 generates a two-dimensional spaceS (FIG. 2 c) according to the first image window W₁₁ and the secondimage window W₁₁′. For example, the first image window W₁₁ contains twopointer images I₈₁ and I₈₂ while the second image window W₁₁′ containsonly one pointer image I′ (Step S₁₃). Next, the processing unit 12obtains positions of the pointers 81 and 82 with respect to the touchsurface 10 using the positioning method of the present invention. Theprocessing unit 12 now respectively maps the pointers 81 and 82 to thepointer images 81′ and 82′ on the two-dimensional space S. In addition,the first image sensor 11 and the second image sensor 11′ arerespectively mapped to mapping positions (0,0) and (960,0) on thetwo-dimensional space S. It is appreciated that, mapping positions ofthe image sensors on the two-dimensional space S are determinedaccording to locations of the image sensors disposed on the touchsurface 10.

Next, the processing unit 12 connects a mapping position (0,0) of thefirst image sensor 11 on the two-dimensional space S with mappingpositions of two outermost edges E₈₁ and E₈₂ of the pointer images I₈₁and I₈₂ in the first image window W₁₁ on the two-dimensional space S soas to form a first edge line L₁ and a second edge line L₂; and connectsa mapping position (960,0) of the second image sensor 11′ on thetwo-dimensional space S with mapping positions of two outermost edgesE₈₁′ and E₈₂′ of the pointer image I′ in the second image window W₁₁′ onthe two-dimensional space S so as to form a third edge line L₃ and afourth edge line L₄ (Step S₁₄). The processing unit 12 then calculatesfour first internal bisectors V₁ to V₄ of a quadrilateral ADBC formed bythe first edge line L₁ to the fourth edge line L₄, wherein the firstinternal bisector V₁ may be obtained by using the vectors {right arrowover (AD)} and {right arrow over (AC)}. Similarly, internal bisectors V₂to V₄ may be obtained in the same way (Step S₁₅).

Next, the processing unit 12 connects a mapping position (0,0) of theimage sensor acquiring more pointer images (i.e. the first image sensor11 herein) on the two-dimensional space S with mapping positions (i.e.centers C₈₁ and C₈₂ of the pointer images) of a predetermined point(e.g. center pointer or center of weight) of the pointer images in thefirst image window W₁₁ on the two-dimensional space S to form two firstpoison lines PL₁ and PL₂ (Step S₁₆). Then, the processing unit 12defines four cross points of the first position lines PL₁, PL₂ and thefirst internal bisectors V₁ to V₄ as four first possible positions P₁ toP₄; wherein two first possible positions associated with the twointernal bisectors of two opposite corners of the quadrilateral ADBC maybe defined as a pair of first possible positions. For example, P₁ and P₂may be defined as a pair of first possible positions and P₃ and P₄ maybe defined as another pair of first possible positions (Step S₁₇).Finally, the processing unit 12 compares the first possible positions P₁to P₄ with a pair of previous correct positions determined in a previoussample time “t−1” of the first image sensor 11 and the second imagesensor 11′ so as to determine a pair of current correct positions (StepS₁₈). For example in an embodiment, the characteristic such as adistance, a moving direction, a moving speed or the like of the pair ofprevious correct positions and two pairs of first possible positions P₁,P₂ and P₃, P₄ may be respectively compared. When the pair of previouscorrect positions has a shortest distance, a closest moving direction ora closest moving speed with one pair of the first possible positions,the pair of first possible positions is identified as the currentcorrect positions, such as P₃ and P₄ herein. In another embodiment, thefour first possible positions P₁ to P₄ may be respectively compared withthe pair of previous correct positions to obtain two current correctpositions.

It is appreciated that, some steps shown in FIG. 2 d may be combinedtogether and the steps shown therein are only for illustrating theimplementation of the positioning method of the present invention ratherthan limitations to the present invention. For example, the process ofobtaining the quadrilateral in Step S₁₅ may be performed in Step S₁₄.

Please referring to FIG. 3, it shows a schematic diagram of the touchsystem 1′ according to the second embodiment of the present inventionincluding a touch surface 10, a first image sensor 11, a second imagesensor 11′, a third image sensor 11″ and a processing unit 12. Thedifference between this embodiment and the first embodiment is that thetouch system 1′ includes three image sensors in this embodiment.Similarly, the processing unit 12 processes the image windows acquiredby the image sensors to accordingly generate a two-dimensional space.The positioning method of the present invention is implemented byperforming coordinate operation and vector arithmetic on thetwo-dimensional space. It is appreciated that, locations of the firstimage sensor 11, the second image sensor 11′ and the third image sensor11″ are not limited to those shown in FIG. 3. For example, the thirdimage senor 11″ may also be disposed at lower left corner.

Please referring to FIG. 4 a, it shows a schematic diagram of thepositioning method for a touch system according to a first aspect of thesecond embodiment of the present invention, in which the processing unit12 generates a two-dimensional space S according to the image windowsacquired by all image sensors and four corners of the two-dimensionalspace S are assumed as (0,0), (X,0), (0,Y) and (X,Y). This aspect isapplied to the case that numbers of pointer images in the image windowsacquired by two image sensors of the touch system 1′ are smaller thanthat acquired by the rest image sensor. For example, the image windowsacquired by the first image sensor 11 and the second image sensor 11′include only one pointer image and the image window acquired by thethird image sensor 11″ includes two pointer images. This aspect isconfigured to obtain two pairs of possible positions or a pair ofcurrent correct positions.

FIG. 4 b shows a flow chart of the positioning method according to thepresent aspect including the steps of: respectively acquiring an imagewindow with three image sensors (Step S₂₁); identifying numbers ofpointer images in the image windows (Step S₂₂); generating atwo-dimensional space according to the three image windows when thenumbers of pointer images in two of the image windows are smaller thanthat in the rest image window (Step S₂₃); connecting mapping positionsof two image sensors acquiring fewer pointer image with mappingpositions of two outermost edges of the pointer images in the imagewindows acquired by the same two image sensors on the two-dimensionalspace to form four edge lines (Step S₂₄); calculating four secondinternal bisectors of a quadrilateral formed by the edge lines (StepS₂₅); connecting a mapping position of the image sensor acquiring morepointer images with mapping positions of a predetermined point of thepointer images in the image window acquired by the same image sensor onthe two-dimensional space to form two second position lines (Step S₂₆);defining cross points of the second position lines and the secondinternal bisectors as second possible positions (Step S₂₇); andcomparing the second possible positions with a pair of previous correctpositions to obtain a pair of current correct positions (Step S₂₈),wherein the pair of previous correct positions is determined in aprevious sample time of the first image sensor to the third imagesensor.

Please referring to FIGS. 4 a and 4 b, the image sensors 11, 11′ and 11″respectively acquire an image window at a sample time “t”, and two ofthe acquired image windows contain only one pointer image (Step S₂₁). Inthe meanwhile, it is assumed that the image windows respectivelyacquired by the image sensors 11, 11′ and 11″ at a sample time “t−1” allinclude two pointer images. That is, one of the pointers 81 and 82 doesnot block the other with respect to any image sensor at the sample time“t−1”.

The processing unit 12 identifies numbers of pointer images in the imagewindows (Step S₂₂). When the processing unit 12 identifies that thenumbers of pointer images in two of the image windows are smaller thanthat in the rest image window, the processing unit 12 generates atwo-dimensional space S according to the three image windows. Forexample, the image windows acquired by the first image sensor 11 and thesecond image sensor 11′ contain only one pointer image while the imagewindow acquired by the third image sensor 11″ contains two pointerimages (Step S₂₃). The processing unit 12 now maps the pointers 81 and82 to the pointer images 81′ and 82′ on the two-dimensional space S. Inaddition, the first image sensor 11, the second image sensor 11′ and thethird image sensor 11″ are respectively mapped to mapping positions(0,0), (X,0) and (X,Y) on the two-dimensional space S. Similarly,mappings positions of the image sensors on the two-dimensional space Sare determined according to locations of the image sensors disposed onthe touch surface 10.

Next, the processing unit 12 connects mapping positions (0,0) and (X,0)of two image sensors acquiring fewer pointer image (i.e. the first imagesensor 11 and second image sensor 11′ herein) respectively with mappingpositions of two outermost edges of the pointer image in the imagewindows acquired by the same two image sensors on the two-dimensionalspace S to form four edge lines L₁ to L₄ (Step S₂₄). The processing unit12 calculates four second internal bisectors V₁ to V₄ of a quadrilateralADBC formed by the first edge line L₁ to the fourth edge line L₄ (StepS₂₅). The processing unit 12 connects a mapping position (X,Y) of theimage sensor acquiring more pointer images (i.e. the third image sensor11″) with mapping positions C₈₁ and C₈₂ of a predetermined point (e.g.center point or center of weight) of the pointer images in the imagewindow acquired by the same image sensor on the two-dimensional space Sto form two second position lines PL₁ and PL₂ (Step S₂₆). The processingunit 12 defines four cross points of the second position lines PL₁, PL₂and the second internal bisectors V₁ to V₄ as four second possiblepositions P₁ to P₄ (Step S₂₇).

In this aspect, the processing unit 12 may obtain a pair of currentcorrect positions by comparing the second possible positions P₁ to P₄obtained in this aspect with other possible positions, which will beobtained in other aspects hereinafter; or by comparing the secondpossible positions P₁ to P₄ with a pair of previous correct positions(as illustrated in the first embodiment) determined in a previous sampletime “t−1” of the first image sensor 11 to the third image sensor 11″(Step S₂₈). For example, two of the second possible positions P₁ to P₄having shortest distances, closest moving directions or moving speedswith respect to the pair of previous correct positions are identified asthe pair of current correct positions, such as P₁ and P₂ herein.

Please referring to FIG. 4 c, it shows a schematic diagram of thepositioning method for a touch system 1′ according to a second aspect ofthe second embodiment of the present invention. This aspect is alsoapplied to the case that numbers of pointer images in the image windowsacquired by two image sensors of the touch system 1′ are smaller thanthat acquired by the rest image sensor. This aspect is configured toobtain two pairs of possible positions or a pair of current correctpositions.

FIG. 4 d shows a flow chart of the positioning method according to thepresent aspect including the steps of: respectively acquiring an imagewindow with three image sensors (Step S₃₁); identifying numbers ofpointer images in the image windows (Step S₃₂); generating atwo-dimensional space according to the three image windows when thenumbers of pointer images in two of the image windows are smaller thanthat in the rest image window (Step S₃₃); connecting a mapping positionof the image sensor acquiring more pointer images with mapping positionsof two outermost edges of the pointer images in the image windowacquired by the same image sensor on the two-dimensional space to formtwo edge lines, and connecting a mapping position of one of two imagesensors acquiring fewer pointer image with mapping positions of twooutermost edges of the pointer image in the image window acquired by thesame image sensor on the two-dimensional space to form another two edgelines (Step S₃₄); calculating four internal bisectors of a quadrilateralformed by the four edge lines (Step S₃₅); connecting a mapping positionof the image sensor acquiring more pointer images with mapping positionsof a predetermined point of the pointer images in the image windowacquired by the same image sensor on the two-dimensional space to formtwo position lines (Step S₃₆); defining cross points of the positionlines and the internal bisectors as possible positions (Step S₃₇); andcomparing the possible positions with a pair of previous correctpositions to obtain a pair of current correct positions (Step S₃₈),wherein the pair of previous correct positions is determined in aprevious sample time of the first image sensor to the third imagesensor.

Please referring to FIGS. 4 c and 4 d together, the image sensors 11,11′ and 11″ respectively acquire an image window at a sample time “t”,and two of the acquired image windows contain only one pointer image(Step S₃₁). In the meanwhile, it is assumed that image windowsrespectively acquired by the image sensors 11, 11′ and 11″ at a sampletime “t−1” all include two pointer images.

The processing unit 12 identifies numbers of pointer images in the imagewindows (Step S₃₂). When the numbers of pointer images in two of theimage windows are identified to be smaller than that in the rest imagewindow, the processing unit 12 generates a two-dimensional space Saccording to the three image windows (Step S₃₃), wherein the pointers 81and 82 are respectively mapped to the pointer images 81′ and 82′ on thetwo-dimensional space S.

Next, the processing unit 12 obtains possible positions (Steps S₃₄ toS₃₇) or a pair of current correct positions (Step S₃₈) according to theimage sensor acquiring more pointer images (i.e. the third image sensor11″ herein) and one of the two image sensors acquiring fewer pointerimage (i.e. the first image sensor 11 or the second image sensor 11′herein) by using the method illustrated in the first embodiment, anddetails thereof were already illustrated in the first embodiment andthus will not be repeated herein.

In this aspect, the processing unit 12 may compare the possiblepositions obtained according to a current frame with the first possiblepositions of the first embodiment or the second possible positions ofthe first aspect of the second embodiment to obtain a pair of currentcorrect positions, such as comparing shortest distances between thosepossible positions. Or the processing unit 12 may compare the possiblepositions obtained in this aspect with a pair of previous correctpositions determined in a previous sample time “t−1” of the first imagesensor 11 to third image sensor 11″ to obtain a pair of current correctpositions.

Please referring to FIGS. 5 a to 5 d, they show schematic diagrams ofthe positioning method for a touch system 1′ according to a third aspectof the second embodiment of the present invention, in which theprocessing unit 12 generates a two-dimensional space S according to theimage windows acquired by all image sensors and four corners of thetwo-dimensional space S are assumed as (0,0), (X,0), (0,Y) and (X,Y).This aspect is applied to the case that numbers of pointer images in theimage windows acquired by two image sensors of the touch system 1′ arelarger than that acquired by the rest image sensor. For example, theimage windows acquired by the first image sensor 11 and the third imagesensor 11″ contain two pointer images while the image window acquired bythe second image sensor 11′ contains only one pointer image. This aspectis configured to obtain two pairs of possible positions or a pair ofcurrent correct positions.

FIG. 5 e shows a flow chart of the positioning method according to thepresent aspect including the steps of: respectively acquiring an imagewindow with three image sensors (Step S₄₁); identifying numbers ofpointer images in the image windows (Step S₄₂); generating atwo-dimensional space according to the three image windows when thenumbers of pointer images in two of the image windows are larger thanthat in the rest image window (Step S₄₃); connecting a mapping positionof one of two image sensors acquiring more pointer images with mappingpositions of two outermost edges of the pointer images in the imagewindow acquired by the same image sensor on the two-dimensional space toform two edge lines, and connecting a mapping position of the imagesensor acquiring fewer pointer image with mapping positions of twooutermost edges of the pointer image in the image window acquired by thesame image sensor on the two-dimensional space to form another two edgelines (Step S₄₄); calculating four third internal bisectors of aquadrilateral formed by the four edge lines (Step S₄₅); connecting amapping position of one of two image sensors acquiring more pointerimages with mapping positions of a predetermined point of the pointerimages in the image window acquired by the same image sensor on thetwo-dimensional space to form two third position lines (Step S₄₆);defining cross points of the third position lines and the third internalbisectors as third possible positions (Step S₄₇); and comparing thethird possible positions with a pair of previous correct positions toobtain a pair of current correct positions (Step S₄₈), wherein the pairof previous correct positions is determined in a previous sample time ofthe first image sensor to the third image sensor.

Please referring to FIGS. 5 a and 5 e together, the image sensors 11,11′ and 11″ respectively acquire an image window at a sample time “t”,and one of the acquired image windows contains only one pointer image(Step S₄₁). In the meanwhile, it is assumed that image windowsrespectively acquired by the image sensors 11, 11′ and 11″ at a sampletime “t−1” all include two pointer images.

The processing unit 12 identifies numbers of pointer images in the imagewindows (Step S₄₂). When the processing unit 12 identifies that thenumbers of pointer images in two of the image windows are larger thanthat in the rest image window, the processing unit 12 generates atwo-dimensional space S according to the three image windows (Step S₄₃).The processing unit 12 respectively maps the pointers 81 and 82 to thepointer images 81′ and 82′ on the two-dimensional space S. In addition,the first image sensor 11, the second image sensor 11′ and the thirdimage sensor 11″ are respectively mapped to mapping positions (0,0),(X,0) and (X,Y) on the two-dimensional space S.

Next, the processing unit 12 connects a mapping position (0,0) or (X,Y)of one of two image sensors acquiring more pointer images (i.e. thefirst image sensor 11 in FIGS. 5 a and 5 b; the third image sensor 11″in FIGS. 5 c and 5 d) with mapping positions of two outermost edges ofthe pointer images in the image window acquired by the same image sensoron the two-dimensional space S to form two edge lines L₁ and L₂, andconnects a mapping position (X,0) of the image sensor acquiring fewerpointer image (i.e. the second image sensor 11′) with mapping positionsof two outermost edges of the pointer image in the image window acquiredby the same image sensor on the two-dimensional space S to form anothertwo edge lines L₃ and L₄ (Step S₄₄). Then, the processing unit 12calculates four third internal bisectors V₁ to V₄ of a quadrilateralADBC formed by the four edge lines L₁ to L₄ (Step S₄₅). The processingunit 12 connects a mapping position (0,0) or (X,Y) of one of two imagesensors acquiring more pointer images (i.e. the third image sensor 11″in FIGS. 5 a and 5 d; the first image sensor 11 in FIGS. 5 b and 5 c)with mapping positions C₈₁ an C₈₂ of a predetermined point (e.g. centerpoint or center of weight) of the pointer images in the image windowacquired by the same image sensor on the two-dimensional space S to formtwo third position lines PL₁ and PL₂ (Step S₄₆). Four cross points P₁ toP₄ of the third position lines PL₁, PL₂ and the third internal bisectorsV₁ to V₄ are defined as third possible positions (Step S₄₇).

In this aspect, the processing unit 12 may compare the third possiblepositions P₁ to P₄ with the first possible positions of the firstembodiment, the second possible positions of the first aspect of thesecond embodiment or the possible positions of the second aspect of thesecond embodiment to obtain a pair of current correct positions. Or theprocessing unit 12 may compare the third possible positions with a pairof previous correct positions (as illustrated in the first embodiment)determined in a previous sample time “t−1” of the first image sensor 11to the third image sensor 11″ so as to obtain a pair of current correctpositions (Step S₄₈). It is appreciated that, this aspect may alsoobtain two pairs of possible positions according to two image sensorsacquiring different numbers of pointer images (e.g. the first imagesensor 11 and the second image sensor 11′, or the second image sensor11′ and the third image sensor 11″), and details thereof were alreadyillustrated in the first embodiment and thus will not be repeatedherein.

Please referring to FIG. 6 a, it shows a schematic diagram of thepositioning method for a touch system 1′ according to a fourth aspect ofthe second embodiment of the present invention, in which the processingunit 12 generates a two-dimensional space S according to the imagewindows acquired by all image sensors and four corners of thetwo-dimensional space S are assumed as (0,0), (X,0), (0,Y) and (X,Y).This aspect is applied to the case that numbers of pointer images in theimage windows acquired by two image sensors of the touch system 1′ arelarger than that acquired by the rest image sensor. For example, theimage windows acquired by the first image sensor 11 and the second imagesensor 11′ contain two pointer images while the image window acquired bythe third image sensor 11″ contains only one pointer image. This aspectis configured to obtain two pairs of possible positions or a pair ofcurrent correct positions.

FIG. 6 b shows a flow chart of the positioning method according to thepresent aspect including the steps of: respectively acquiring an imagewindow with three image sensors (Step S₅₁); identifying numbers ofpointer images in the image windows (Step S₅₂); generating atwo-dimensional space according to the three image windows when thenumbers of pointer images in two of the image windows are larger thanthat in the rest image window (Step S₅₃); connecting mapping positionsof two image sensors acquiring more pointer images with mappingpositions of a predetermined point of the pointer images in the imagewindows acquired by the same two image sensors on the two-dimensionalspace to form a quadrilateral (Step S₅₄); defining four corners of thequadrilateral as fourth possible positions (Step S₅₅); and comparing thefourth possible positions with a pair of previous correct positions toobtain a pair of current correct positions (Step S₅₆), wherein the pairof previous correct positions is determined in a previous sample time ofthe first image sensor to the third image sensor.

Please referring to FIGS. 6 a and 6 b together, the image sensors 11,11′ and 11″ respectively acquire an image window at a sample time “t”,and one of the acquired image windows contains only one pointer image(Step S₅₁). In the meanwhile, it is assumed that image windowsrespectively acquired by the image sensors 11, 11′ and 11″ at a sampletime “t−1” all include two pointer images.

The processing unit 12 identifies numbers of pointer images in the imagewindows (Step S₅₂). When the numbers of pointer images in two of theimage windows are identified to be larger than that in the rest imagewindow, the processing unit 12 generates a two-dimensional space Saccording to the three image windows (Step S₅₃), wherein the pointers 81and 82 are respectively mapped to the pointer images 81′ and 82′ on thetwo-dimensional space S; and the first image sensor 11, the second imagesensor 11′ and the third image sensor 11″ are respectively mapped tomapping positions (0,0), (X,0) and (X,Y) on the two-dimensional space S.

Next, the processing unit 12 connects mapping positions (0,0) and (X,0)of two image sensors acquiring more pointer images (i.e. the first imagesensor 11 and the second image sensor 11′) respectively with mappingpositions of a predetermined point (e.g. center point or center ofweight) of the pointer images in the image windows acquired by the sametwo image sensors on the two-dimensional space S to form a quadrilateralADBC (Step S₅₄). Four corners of the quadrilateral ADBC are defined asfourth possible positions (Step S₅₅).

In this aspect, the processing unit 12 may compare the fourth possiblepositions P₁ to P₄ obtained in this aspect with the possible positionsobtained in the first embodiment or in every aspect of the secondembodiment to obtain a pair of current correct positions. Or theprocessing unit 12 may compare the fourth possible positions obtained inthis aspect with a pair of previous correct positions (as illustrated inthe first embodiment) determined in a previous sample time “t−1” of thefirst image sensor 11 to the third image sensor 11″ so as to obtain apair of current correct positions (Step S₅₆). It is appreciated that,this aspect may also obtain two pairs of possible positions according totwo image sensors acquiring different numbers of pointer images (e.g.the second image sensor 11′ and the third image sensor 11″), and detailsthereof were already illustrated in the first embodiment and thus willnot be repeated herein.

In a word, the positioning method for a touch system of the presentinvention may obtain a pair of current correct positions by comparingtwo pairs of possible positions in a current frame with a pair ofprevious correct positions in a previous frame; or obtain a pair ofcurrent correct positions by comparing two pairs of possible positionsin a current frame respectively obtained from different embodiments oraspects described above. In the present invention, the previous frame isan effective frame previous to the current frame. For example, if animmediately previous frame of the current frame has a poor image qualityso that it is identified as an invalid frame, the previous frame of thecurrent frame may be the second or the nth frame previous to the currentframe.

In addition, although two pointers are used for illustration in theabove embodiments and aspects, the positioning method of the presentinvention is also applicable to the positioning of more than twopointers.

In addition, in the comparison of possible positions, the presentinvention is not limited to compare a pair of possible positions at thesame time, and every possible position may be sequentially andseparately compared so as to obtain current correct positions. Forexample, four possible positions (P₁, P₂, P₃, P₄) obtained in anyembodiment or aspect above may be respectively compared with anotherfour possible positions (P₁′, P₂′, P₃′, P₄′) obtained in anotherembodiment or aspect. Or positions, moving speeds and/or movingdirections of four possible positions (P₁, P₂, P₃, P₄) obtained in anyembodiment or aspect above may be compared with that of a pair ofprevious correct positions so as to obtain two or a pair of currentcorrect positions.

As mentioned above, as conventional touch systems have the problem ofunable to correctly position a plurality of pointers, the presentinvention further provides a touch system (FIGS. 2 a and 3) and apositioning method therefore (FIGS. 2 d, 4 b, 4 d, 5 e and 6 b) that cancorrectly trace and position two-dimensional coordinates of a pluralityof pointers with respect to a touch system.

Although the invention has been explained in relation to its preferredembodiment, it is not used to limit the invention. It is to beunderstood that many other possible modifications and variations can bemade by those skilled in the art without departing from the spirit andscope of the invention as hereinafter claimed.

1. A positioning method for a touch system, the touch system comprisinga first image sensor and a second image sensor for acquiring imagewindows looking across a touch surface and containing images of twopointers operating above the touch surface, the positioning methodcomprising the steps of: acquiring a first image window with the firstimage sensor; acquiring a second image window with the second imagesensor; identifying numbers of pointer images in the first image windowand the second image window; generating a two-dimensional spaceaccording to the first image window and the second image window when thefirst image window and the second image window contain different numbersof pointer images; connecting, on the two-dimensional space, a mappingposition of the first image sensor with mapping positions of twooutermost edges of the pointer image in the first image window andconnecting, on the two-dimensional space, a mapping position of thesecond image sensor with mapping positions of two outermost edges of thepointer image in the second image window to form a quadrilateral;calculating four first internal bisectors of the quadrilateral; andconnecting, on the two-dimensional space, a mapping position of theimage sensor acquiring more pointer images with mapping positions of apredetermined point of the pointer images in the image window acquiredby the same image sensor to intersect with the first internal bisectorsthereby generating first possible positions.
 2. The positioning methodas claimed in claim 1, further comprising: defining two first possiblepositions associated with the two first internal bisectors of twoopposite corners of the quadrilateral as a pair of first possiblepositions.
 3. The positioning method as claimed in claim 1, wherein apair of previous correct positions of the pointers with respect to thetouch surface was determined in a previous sample time before the firstimage sensor acquires the first image window and the second image sensoracquires the second image window, and the positioning method furthercomprises: comparing the first possible positions with the pair ofprevious correct positions to obtain a pair of current correctpositions.
 4. The positioning method as claimed in claim 3, wherein thecomparison process is to compare a distance, a moving direction and/or amoving speed of the first possible positions with that of the pair ofprevious correct positions.
 5. The positioning method as claimed inclaim 1, wherein the touch system further comprises a third image sensorfor acquiring image windows looking across the touch surface andcontaining images of the two pointers, and the positioning methodfurther comprises: acquiring a third image window with the third imagesensor; identifying the numbers of pointer images in the first, secondand third image windows; mapping the third image window to thetwo-dimensional space when the numbers of pointer images in two of theimage windows are smaller than that in the rest image window;connecting, on the two-dimensional space, mapping positions of two imagesensors acquiring fewer pointer image with mapping positions of twooutermost edges of the pointer image in the image windows acquired bythe same two image sensors to form a quadrilateral; calculating foursecond internal bisectors of the quadrilateral; connecting, on thetwo-dimensional space, a mapping position of the image sensor acquiringmore pointer images with mapping positions of a predetermined point ofthe pointer images in the image window acquired by the same image sensorto intersect with the second internal bisectors thereby generatingsecond possible positions; and comparing the first possible positionswith the second possible positions to obtain a pair of current correctpositions.
 6. The positioning method as claimed in claim 1, wherein thetouch system further comprises a third image sensor for acquiring imagewindows looking across the touch surface and containing images of thetwo pointers, and the positioning method further comprises: acquiring athird image window with the third image sensor; identifying the numbersof pointer images in the first, second and third image windows; mappingthe third image window to the two-dimensional space when the numbers ofpointer images in two of the image windows are larger than that in therest image window; connecting, on the two-dimensional space, a mappingposition of one of two image sensors acquiring more pointer images withmapping positions of two outermost edges of the pointer images in theimage window acquired by the same image sensor and connecting, on thetwo-dimensional space, a mapping position of the image sensor acquiringfewer pointer image with mapping positions of two outermost edges of thepointer image in the image window acquired by the same image sensor toform a quadrilateral; calculating four third internal bisectors of thequadrilateral; connecting, on the two-dimensional space, a mappingposition of one of two image sensors acquiring more pointer images withmapping positions of a predetermined point of the pointer images in theimage window acquired by the same image sensor to intersect with thethird internal bisectors thereby generating third possible positions;and comparing the first possible positions with the third possiblepositions to obtain a pair of current correct positions.
 7. Thepositioning method as claimed in claim 1, wherein the touch systemfurther comprises a third image sensor for acquiring image windowslooking across the touch surface and containing images of the twopointers, and the positioning method further comprises: acquiring athird image window with the third image sensor; identifying the numbersof pointer images in the first, second and third image windows; mappingthe third image window to the two-dimensional space when the numbers ofpointer images in two of the image windows are larger than that in therest image window; connecting, on the two-dimensional space, mappingpositions of two image sensors acquiring more pointer images withmapping positions of a predetermined point of the pointer images in theimage windows acquired by the same two image sensors to form aquadrilateral; defining four corners of the quadrilateral as fourthpossible positions; and comparing the first possible positions with thefourth possible positions to obtain a pair of current correct positions.8. The positioning method as claimed in claim 1, wherein thepredetermined point is a center point or a center of weight of thepointer image.
 9. The positioning method as claimed in claim 5, whereinthe predetermined point is a center point or a center of weight of thepointer image.
 10. The positioning method as claimed in claim 6, whereinthe predetermined point is a center point or a center of weight of thepointer image.
 11. The positioning method as claimed in claim 7, whereinthe predetermined point is a center point or a center of weight of thepointer image.
 12. A positioning method for a touch system, the touchsystem comprising a first image sensor, a second image sensor and athird image sensor for acquiring image windows looking across a touchsurface and containing images of two pointers operating above the touchsurface, the positioning method comprising the steps of: respectivelyacquiring an image window with three image sensors; identifying numbersof pointer images in the image windows; generating a two-dimensionalspace according to the three image windows; executing the followingsteps when the numbers of pointer images in two of the image windows issmaller then that in the rest image window: connecting, on thetwo-dimensional space, mapping positions of two image sensors acquiringfewer pointer image with mapping positions of two outermost edges of thepointer image in the image windows acquired by the same two imagesensors to form a quadrilateral; calculating four second internalbisectors of the quadrilateral; and connecting, on the two-dimensionalspace, a mapping position of the image sensor acquiring more pointerimages with mapping positions of a predetermined point of the pointerimages in the image window acquired by the same image sensor tointersect with the second internal bisectors thereby generating secondpossible positions; and executing the following steps when the numbersof pointer images in two of the image windows is larger then that in therest image window: connecting, on the two-dimensional space, a mappingposition of one of two image sensors acquiring more pointer images withmapping positions of two outermost edges of the pointer images in theimage window acquired by the same image sensor and connecting, on thetwo-dimensional space, a mapping position of the image sensor acquiringfewer pointer image with mapping positions of two outermost edges of thepointer image in the image window acquired by the same image sensor toform a quadrilateral; calculating four third internal bisectors of thequadrilateral; and connecting, on the two-dimensional space, a mappingposition of one of two image sensors acquiring more pointer images withmapping positions of a predetermined point of the pointer images in theimage window acquired by the same image sensor to intersect with thethird internal bisectors thereby generating third possible positions.13. The positioning method as claimed in claim 12, wherein a pair ofprevious correct positions of the pointers with respect to the touchsurface was determined in a previous sample time before the imagesensors acquire the image windows, and the positioning method furthercomprises: comparing the second possible positions with the pair ofprevious correct positions to obtain a pair of current correct positionswhen the numbers of pointer images in two of the image windows aresmaller than that in the rest image window; and comparing the thirdpossible positions with the pair of previous correct positions to obtaina pair of current correct positions when the numbers of pointer imagesin two of the image windows are larger than that in the rest imagewindow.
 14. The positioning method as claimed in claim 12, furthercomprising: selecting two image sensors acquiring different numbers ofpointer images; connecting, on the two-dimensional space, mappingpositions of the two image sensors respectively with mapping positionsof two outermost edges of the pointer images in the image windowsacquired by the same two image sensors to form a quadrilateral;calculating four first internal bisectors of the quadrilateral;connecting, on the two-dimensional space, a mapping position of one ofthe two image sensors acquiring more pointer images with mappingpositions of a predetermined point of the pointer images in the imagewindow acquired by the same image sensor to intersect with the firstinternal bisectors thereby generating first possible positions.
 15. Thepositioning method as claimed in claim 14, further comprising: comparingthe second possible positions with the first possible positions toobtain a pair of current correct positions when the numbers of pointerimages in two of the image windows are smaller than that in the restimage window; and comparing the third possible positions with the firstpossible positions to obtain a pair of current correct positions whenthe numbers of pointer images in two of the image windows are largerthan that in the rest image window.
 16. The positioning method asclaimed in claim 12, wherein when the numbers of pointer images in twoof the image windows are larger than that in the rest image window, thepositioning method further comprising: connecting, on thetwo-dimensional space, mapping positions of two image sensors acquiringmore pointer images with mapping positions of a predetermined point ofthe pointer images in the image windows acquired by the same two imagesensors to form a quadrilateral; defining four corners of thequadrilateral as fourth possible positions; and comparing the thirdpossible positions with the fourth possible positions to obtain a pairof current correct positions.
 17. The positioning method as claimed inclaim 12, wherein the predetermined point is a center point or a centerof weight of the pointer image.
 18. The positioning method as claimed inclaim 14, wherein the predetermined point is a center point or a centerof weight of the pointer image.
 19. The positioning method as claimed inclaim 16, wherein the predetermined point is a center point or a centerof weight of the pointer image.
 20. A touch system, comprising: a touchsurface, wherein a plurality of pointers are operated above the touchsurface to accordingly control the touch system; at least two imagesensors configured to acquire image windows looking across the touchsurface and containing images of the pointers operating above the touchsurface; and a processing unit generating a two-dimensional spaceaccording the image windows acquired by the image sensors, obtaining aquadrilateral and four internal bisectors of the quadrilateral byconnecting mapping positions of the image sensors with mapping positionsof two outermost edges of the pointer image in the image windowsacquired by the image sensors on the two-dimensional space, andconnecting a mapping position of the image sensor acquiring more pointerimages with mapping positions of a predetermined point of the pointerimages in the image window acquired by the same image sensor tointersect with the internal bisectors thereby generating possiblepositions.